Existence of micrometer-scale water droplets at solvent/air interfaces

Exploiting Heteroaggregation to Quantify the Contact Angle of Charged Colloids at Interfaces

We exploit the aggregation between oppositely charged particles to visualize and quantify the equilibrium position of charged colloidal particles at the fluid-water interface. A dispersion of commercially available charge-stabilized nanoparticles was used as the aqueous phase to create oil-water and air-water interfaces. The colloidal particles whose charge was opposite that of the nanoparticles in the aqueous phase were deposited at the chosen fluid-water interface. Heteroaggregation, i.e., aggregation between oppositely charged particles, leads to the deposition of nanoparticles onto the larger particle located at the interface; however, this only occurs on the surface of the particle in contact with the aqueous phase. This selective deposition of nanoparticles on the surfaces of the particles exposed to water enables the distinct visualization of the circular three-phase contact line around the particles positioned at the fluid-water interface. Since the electrostatic association between the nanoparticles and the colloids at interfaces is strong, the nanoparticle assembly on the larger particles is preserved even after being transferred to solid substrates via dip-coating. This facilitates the easy visualization of the contact line by electron microscopy and the determination of the equilibrium contact angle of colloidal particles (θ) at the fluid-water interface. The suitability of the method is demonstrated by the measurement of the three-phase contact angle of positively and negatively charged polystyrene particles located at fluid-water interfaces by considering particles with sizes varying from 220 nm to 8.71 μm. The study highlights the effect of the size ratio between the nanoparticles in the aqueous phase and the colloidal particles on the accuracy of the measurement of θ.

A broad perspective to particle-laden fluid interfaces systems: from chemically homogeneous particles to active colloids

Particles adsorbed to fluid interfaces are ubiquitous in industry, nature or life. The wide range of properties arising from the assembly of particles at fluid interface has stimulated an intense research activity on shed light to the most fundamental physico-chemical aspects of these systems. These include the mechanisms driving the equilibration of the interfacial layers, trapping energy, specific inter-particle interactions and the response of the particle-laden interface to mechanical perturbations and flows. The understanding of the physico-chemistry of particle-laden interfaces becomes essential for taking advantage of the particle capacity to stabilize interfaces for the preparation of different dispersed systems (emulsions, foams or colloidosomes) and the fabrication of new reconfigurable interface-dominated devices. This review presents a detailed overview of the physico-chemical aspects that determine the behavior of particles trapped at fluid interfaces. This has been combined with some examples of real and potential applications of these systems in technological and industrial fields. It is expected that this information can provide a general perspective of the topic that can be exploited for researchers and technologist non-specialized in the study of particle-laden interfaces, or for experienced researcher seeking new questions to solve.

Capturing the value in printed pharmaceuticals - A study of inkjet droplets released from a polymer matrix

The future of personalised combination dosages will rely on the programming and delivery of multiple, separate APIs, their carrier fluids and excipients to a stable matrix, where each will remain separate until it is needed to be released. A recent technique has demonstrated how to print, capture and release materials from a polymer matrix using inkjet printing, a low cost and customisable technique. Droplets of a formulation are delivered to a fluid polymer matrix, where they are imbibed and remain pinned just under the upper surface, held in place by a balance of interfacial energies. Once the surrounding matrix cures and solidifies, the coating or matrix has trapped the formulation, but each drop has a small opening or pore to the outside that will allow delivery through diffusion. However, while the trapping mechanism has been explored in detail, to-date the release involved in this delivery has never been studied or quantified to the same level. Here we show a first study to quantify the release of a model system from a polymer matrix. An aqueous fluorescein solution is delivered and trapped, with release demonstrated to an agarose gel and aqueous environments. The work reveals that the balance of interfacial tensions prevents a reliable release until low concentrations of surfactant are included. This provides a route forward to further explore stabilising combinations of drugs within one material using a digitally controlled and affordable technique.

Direct-writing microporous polymer architectures - print, capture and release

In the nature-inspired breath figure method, rafts of condensed water droplets self-organise and imprint into a permanent microporous polymer structure. This could have exciting applications in drug delivery, tissue engineering and sensors but it is extremely difficult to control or functionalise the final structure. Here, we show direct-writing of droplets onto fluid surfaces by inkjet printing as a breakthrough to dial-in a required pattern, structure and function into the polymer film.

Particle contact angles at fluid interfaces: pushing the boundary beyond hard uniform spherical colloids

Micro and nanoparticles at fluid interfaces have been attracting increasing interest in the last few decades as building blocks for materials, as mechanical and structural probes for complex interfaces and as models for two-dimensional systems. The three-phase contact angle enters practically all aspects of the particle behavior at the interface: its thermodynamics (binding energy to the interface), dynamics (motion and drag at the interface) and interactions with the interface (adsorption and wetting). Moreover, many interactions among particles at the interface also strongly depend on the contact angle. These concepts have been extensively discussed for non-deformable, homogeneous and mostly spherical particles, but recent progress in particle synthesis and fabrication has instead moved in the direction of producing more complex micro and nanoscale objects, which can be responsive, deformable, heterogenous and/or anisotropic in shape, surface chemistry and material properties. These new particles have a much greater potential for applications and new science, and the study of their behavior at interfaces has only very recently started. In this paper, we critically review the current state of the art of the experimental methods available to measure the contact angle of micro and nanoparticles at fluid interfaces, indicating their strengths and limitations. We then comment on new particle systems that are currently attracting increasing interest in relation to their adsorption and assembly at fluid interfaces and discuss if and which ones of the current techniques are suited to investigate their properties at interfaces. Based on this discussion, we will finally try to indicate a direction in which new experimental methods should develop in the future to tackle the new challenges posed by the novel types of particles that more and more often are used at interfaces.

Taming Self-Organization Dynamics to Dramatically Control Porous Architectures

We demonstrate templating of functional materials with unexpected and intricate micro- and nanostructures by controlling the condensation, packing, and evaporation of water droplets on a polymer solution. Spontaneous evaporation of a polymer solution induces cooling of the liquid surface and water microdroplet condensation from the ambient vapor. These droplets pack together and act as a template to imprint an entangled polymer film. This breath figure (BF) phenomenon is an example of self-organization that involves the long-range ordering of droplets. Equilibrium-based analysis provides many insights into contact angles and drop stability of individual drops, but the BF phenomenon remains poorly understood thus far, preventing translation to real applications. Here we investigate the dynamics of this phenomenon to separate out the competing influences and then introduce a modulation scheme to ultimately manipulate the water vapor-liquid equilibrium independently from the solvent evaporation. This approach to BF control provides insights into the mechanism, a rationale for microstructure design, and evidence for the benefits of dynamical control of self-organization systems. We finally present dramatically different porous architectures from this approach reminiscent of microscale Petri dishes, conical flasks, and test tubes.